The present invention relates to a gas sensor installed in an exhaust system of an automotive internal combustion engine to detect an oxygen concentration in the exhaust gas, or an air-fuel ratio, or the like.
The present invention relates to a gas sensing element used for controlling an air-fuel ratio of an internal combustion engine and a method for manufacturing the gas sensing element.
In general, to control the air-fuel ratio, a gas sensor is installed in an exhaust system of an automotive internal combustion engine.
The gas sensor comprises a gas sensing element provided at its front end for detecting an oxygen concentration. The gas sensing element comprises a solid electrolytic sintered body having oxygen ion conductance, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The measured gas side electrode is covered by a porous electrode protective layer.
In many cases, the electrode protective layer is a ceramic coating layer, or a double layer consisting of a ceramic coating layer and a γ-Al2O3 layer provided on this ceramic coating layer.
According to this type of gas sensing element, a measured gas reaches a measured gas side electrode through the ceramic coating layer or the double layer of the ceramic coating layer and the γ-A1203 layer. The bas sensing element produces a sensor output.
Recent radically changing circumstances, such as enhancement of emission control laws and regulations as well as requirements for high power internal combustion engines, has forced automotive manufacturers to develop automotive engines capable of precisely controlling combustion.
To realize this, it is essentially important to provide excellent gas sensors having sensing properties stable under severe operating conditions and durable for a long-term use.
FIG. 5 shows a characteristic curve representing a relationship between air-fuel ratio and voltage, as important sensor output characteristics of a gas sensing element used for combustion control of an internal combustion engine. In FIG. 5, point λ is referred to as a specific air-fuel ratio where the voltage undergoes steep changes. In FIG. 5, a reference voltage is a criteria used for judging whether a fuel injection amount should be increased or decreased in the combustion control of an internal combustion engine. In general, the reference voltage is set to 0.45V.
More specifically, when a sensor output is larger than the reference voltage, the fuel injection amount is reduced to form an air/fuel mixture whose air-fuel ratio is shifted to a lean side. On the contrary, when a sensor output is less than the reference voltage, the fuel injection amount is increased to form a relatively rich air/fuel mixture. Through such a feedback control, the air-fuel ratio of the controlled engine can be always kept in a window of a ternary catalyst.
Accordingly, to precisely perform the air-fuel ratio control, it is essentially important to stabilize the point λ (hereinafter, referred to as control λ).
In other words, the control λ should be stable during a long-term use of a gas sensing element and should be constant regardless of any environmental change of the gas sensing element.
When a gas sensing element is installed in an exhaust system of an internal combustion engine, a sensor output is produced in the following manner.
First, an exhaust gas containing unburnt components reaches a measured gas side electrode. Then, an equilibrium oxygen concentration is obtained through a catalytic reaction caused on the measured gas side electrode. The sensor output is produced as a signal representing a difference between the equilibrium oxygen concentration thus obtained and an oxygen concentration in the air serving as a reference gas.
Accordingly, it becomes possible to increase the measuring accuracy of a gas sensing element when an electrode having excellent activity is used as a measured gas side electrode of a gas sensing element.
The following is a method for activating a measured gas side electrode disclosed in Unexamined Japanese patent publication No. 10-104194.
First, a measured gas side electrode is formed on a surface of a solid electrolytic body by baking in the air at the temperature range from 1,000° C. to 1,400° C. Then, a heat treatment is applied to the measured gas side electrode thus formed in an atmosphere containing H2.
Subsequently, a heat treatment in an inert atmosphere and a heat treatment in a non-oxidative atmosphere including moisture vapor are applied to the measured gas side electrode.
By combining these treatments, the catalytic activity of the measured gas side electrode can be enhanced.
However, according to the above-described conventional method, it was difficult to provide a gas sensing element having a measured gas side electrode which can assure a sufficiently stable control λ even in a severe high-temperature environment or in a poisonous environment containing Si compounds.